Ketu is a shadow planet. Most astrologers consider Ketu as malefic(bad) planet though Ketu is among the two planets which can provide moksha to an individual, the other being Jupiter. Below are few points where native could be troubled by ketu.

1. A nihilistic individual is likely to be under the influence of Ketu.
2. Often some natives get detached and leave their home and family. They are very likely to be under influence of Ketu at that time.
3. Natives who cannot control their anger and tend to physically and emotionally hurt their loved ones could have a bad Ketu along with Mars.
4. Those who go bald or get gray hair at a very young age are likely to have Ketu in ascendant or along with ascendant lord or Moon.
5. Ketu can also cause wrinkles and other signs of ageing very early in life.
6. Placement of Ketu with Moon in a horoscope leads to a reclusive and solitary individual. Such anti-social behavior is most likely due to bad effects of Ketu.
7. Placement of Ketu with Saturn tends to frustrate the individual and leads to emotional outbursts on few occasions. Both of them in Eight house could lead to divorce and sour relationship with spouse and in-laws.
8. Place of Ketu with Mars is equally bad and produces Anagaraka Yoga which increases anger and frustration in an individual.

     Ketu influenced natives are angry, tend to be recluse and hate social customs and bonding. They may also show signs of ageing at a very young age. Prayers to Lord Ganesha are ideal for such natives. Lord Ganesha is the lord of wisdom and impulses. 

     Ketu natives could even pray to Lord Krishna. Krishna was married to 16,108 wives, lived in a golden palace and was served 56 kinds of delicacies on daily basis. Yet in the mind he lived life of an ascetic whilst totally blending in with society. Bhagvad Gita is a testament to his knowledge and ascetic school of thoughts.

     Maharishi(great sage) Ved Vyas composed the Ketu Japa Mantra which can lessen the effects of bad Ketu.

Ketu Japa Mantra

पलाशपुष्पसंकाशं तारकाग्रह मस्तकम् |
रौद्रंरौद्रात्मकं घोरं तं केतुं प्रणमाम्यहम् ||

Palaasha-pushpa-samkasham Taaraka-graha-mastakam
Raudram-raudratmakam Ghoram tam Ketum Pranamamyaham

Ketu Gayatri Mantra

om aswadhwajaaya vidmahae soola hastaaya dheemahi tanno ketu: prachodayaat

     Natives with weak Ketu should try light exercises and pranayam. They should also take part in outdoor sports like soccer, cricket, volley ball and other sports where teamwork and understanding increases with others. Any outdoor activity or sport which helps communicate, encourages involvement and build a positive character are advised. By all means natives with weak Ketu should avoid gaming consoles and excessive use of mobile apps as they alienate the native from the society.

     Natives with weak Ketu should regularly feed dogs. Also they should give black clothes and other black items to less fortunate souls.

     But perhaps the most important thing to aid a weak Ketu is to learn to accept faults and inaccuracies in those around and try to live a harmonious existence with them. There can be thousands problems with society and people around but it is not necessary to point them out and hurt their feelings. Like Lord Krishna one should try to keep a happy and cheerful face while remaining an ascetic by mind. That is the sure shot way to win over weak Ketu in a horoscope.